Various types of signaling networks are known in the art. The present applicant/assignee currently markets a system known under the trademark SPIDERALERT, which provides personal alert services within a protected region, such as, for example for students and staff on university campuses, employees in a corporate facility, medical staff and patients, correctional officers, and users of large parking lots and garages. Once activated by a user, the SPIDERALERT system indicates both the identity and the location of the person requesting assistance.
The SPIDERALERT system is normally based on a user-actuable portable RF transmitter providing a user-identifying signal which is sensed by one or more RF receivers distributed throughout the protected region.
It has been found that when the protected region comprises a multiplicity of closely spaced together, individually walled-off sub-regions, such as hospital rooms or offices, each of which contains a receiver, it is often difficult to pinpoint the individual sub-region from which the alarm signal is being transmitted, due to the fact that RF signals readily pass through most interior partitions in a building and are detected by more than one RF receiver. Failure to pinpoint the individual room from which an alarm signal is being transmitted, could cause inconvenience and possibly critical delay in emergency situations.
U.S. Pat. No. 4,630,035 to Motorola, Inc. describes an alarm system having alarm transmitter identification codes and acoustic ranging. The location of an alarm is determined by sensing the time of arrival of two different signals and further requires that one signal have a propagation speed through air which is substantially different from that of the other signal.
U.S. Pat. No. 4,347,501 to Ericsson describes an installation for transmitting alarm signals wherein portable alarm devices transmit a coded message which includes coded information as to the location of the portable alarm device. This code is supplied to the portable alarm devices by local fixed transmitters located in each area.